Bug in 'codecvt<_InternT, _ExternT, encoding_state>::do_unshift' (non-standard extension)

[Paolo Carlini]

Kristian Spangsege wrote:
> The 'unshift' method of the 'iconv' specialization of 'codecvt' in
> "ext/codecvt_specializations.h" does not work correctly.
>   
Probably, you may want to file a PR to keep track of  this issue. In any
case, note that the iconv specialization definitely isn't ready for
prime time and we are certainly not suggesting any use beyond
experimenting. Sorry if this is not clear from the documentation,
hopefully we can make progress on that pretty soon.

Paolo.